The present invention relates to orthopedic implantation, particularly implantation of prosthetic devices to repair or replace hard tissue of warmblooded mammals, e.g., bones and joints of humans and animals. The process utilizes a bone cement for fixation of the prosthesis.
One of the inherent problems of orthopedic implantation is the fixation and the maintenance of a stable interface between the device and the host tissue. Polymethylmethacrylate based bone cement has been widely used to fix the implant. The bone cement fixation creates two interfaces: cement/bone and cement/implant. According to an earlier report the incidence of loosening for the femoral prostheses of hip joint arthroplasties were evenly divided at about 10% and 11% for cement/implant and cement/bone interfaces, respectively. The cement/implant interface loosening can be minimized by pre-coating with bone cement or polymethylmethacrylate polymer. Pre-coating can achieve a good bonding between the "cement" and prosthesis during the manufacturing process. During surgery, the freshly doughed cement adheres well to the pre-coated cement.
The problems at the bone/cement interface cannot be easily overcome since these problems arise from the intrinsic properties of the bone cement as well as extrinsic factors such as cementing technique. The toxicity of the monomer, inherent weakness of the cement as a material, and inevitable inclusion of the pores can contribute to the problem of loosening at the bone/cement interface.
The bone/cement interface strength may be enhanced by the bone ingrowth into the cement. Bone cement can be used for immediate fixation yet provide tissue ingrowth space later by incorporating resorbable particles (such as demineralized or deproteinized bone).
In summary, potential advantages of the idea of using resorbable particle impregnated (polymethyl methacrylate) bone cement for the fixation of orthopedic implant for joint replacements are (1) Immediate fixation of the prosthesis and (2) Long term fixation of the prosthesis when the new tissues (bones) to replace the resorbable particles. However, there a are few inherent problems associated with the system: (1) The impregnation of the particles changes the physical properties for the worse in comparison with the original cement such as increased viscosity, making it difficult to mix and inject the cement, decreased mechanical strength, etc. (2) A more serious problem is the "unresorbable" particles due to the coating by the liquid monomer(methylmethacrylate) during mixing, which will result in preventing the resorption of particles. Earlier experiments on the bone morphogenic protein (BMP) resorbable particle impregnated cement did not fare as well as expected due to this problem.
A previous invention of mine relates to pre-coating of the prosthetic implant with bone cement composition in an effort to improve the bond at the interface of the implant and the bone cement composition, see Park, U.S. Pat. No. 4,491,987 issued Jan. 9, 1985 and entitled METHOD OF ORTHOPEDIC IMPLANTATION AND IMPLANT PRODUCT. The process and implant there described has met with a reasonable degree of success. However, this prior invention deals with the interface of the implant and the bone cement. There is also another interface of serious concern, namely, the interface of the bone and the bone cement. This interface is along the cavity made in the bone adapted for receipt of the implant. Typically, this cavity is referred to as the "prepared bed". The concept of placing resorbable particles in the bone cement composition so that the body of the patient resorbs the particles, leaving a porous structure for new bone growth tissue, is known. However, the prior practices involve mixing resorbable particles with the entire bone cement composition. This has some inherent problems in that it substantially weakens the cement, particularly with regard to the relationship of the bonding cement to the implant.
The present invention involves elimination or decrease of the problems associated with the bone cement fixation of a prosthesis by using resorbable particle delivering systems to allow the advantages of strong bond between pure bone cement and the implant, and at the same time, allows the advantages of strong bonding between the bone and a mixture of bone cement and resorbable particles. As a result, adjacent to the interface between the bone and cement, resorbable particles are resorbed over time and replaced with new ingrown bone tissue, further enhancing the bonding at this interface.
Accordingly, it is a primary objective of the present invention to provide an improved delivery system for inserting into patients prosthetic devices which must be fixed to the patients bone by a cement fixation process.
Another object of the present invention is to provide a delivery system which allows all of the advantages of the pure cement bonding to the implant, and as well, all of the advantages of bonding of a cement resorbable particle composition in the interface of the patient's bone in the prepared bed of the bone.
A yet further object of the present invention is to provide numerous delivery systems which accomplish each of the above achieved objectives.
The method and manner of accomplishing all of the above objectives, as well as others, will become apparent from the detailed description of the invention which follows hereinafter.